There are other servers, for example our GTR server, which stays empty, because people rather stay in the multiclass server.
That's more due to the attraction of a full server, and the fact that GTRs are still available on the S2 server than anything else. We saw a few nights ago a server with STD@AS7R was more popular than cargame (almost 40 connections) - no nonsense, no restriction on LFSEI/SR, single class racing, albeit the combo is practice for a league starting soon, but it does show what can happen with popular server, it usually attracts more people.
But... that's the whole purpose of multiclass racing. And the S2 server is ... a multiclass server!!
But I don't think anyone should deliberately create multiclass racing unless it can be helped. I understand why CG is MC, it was the (IHR) replacement after all, and to incentivise the points/SR system you need other cars that people can aim to get. But in real life the purpose of MC racing is to provide a big enough grid out of classes which do not have enough to form their own race, not to simply create passing by faster cars, i.e. LMP1+2 really needs GTE-Pro/Am at Le Mans otherwise you'd only have 28 of the 56 cars on a 13 km track.
I mean, how many classes do you have which could not actually hold their own in a single or two class race? STD, TBO, LX4, LRF, UFB, NGT, GT2 - all of these (bar LRF perhaps) could host their own server instead of having to share the same 32 car grid, instead of being in a class with a handful of other cars. Do you have any statistics about class usage over the course of a day?
Also, as you saw with Norbert Wolf, the S2 server is perhaps not the most friendly for newer guys, and at the end of the day, LFS needs as many new players as we can get, right? They go to the popular server but have to drive an STD with maybe one or two others, at the mercy of all the faster cars.
Personally I'd like to see each class rotated for a week, one class per day, to see what happens.
As for leaving space in the grid, pretty sure it can't be done in LFS at the moment.